Trip systems are designed to respond to power faults detected in circuit breakers. Most simple trip systems employ an electromagnet to trip the circuit in response to short circuit or overload faults. The electromagnet provides a magnetic field in response to the current flowing through the breaker. When the current level increases beyond a predetermined threshold, the magnetic field "trips" a mechanism which causes a set of circuit breaker contacts to release, thereby "breaking" the circuit path.
Many simple trip systems also employ a slower responding bi-metallic strip, which is useful for detecting a more subtle overload fault. This is because the extent of the strip's deflection represents an accurate thermal history of the circuit breaker and, therefore, even slight current overloads. Generally, the heat generated by the current overload will cause the bi-metallic strip to deflect into the tripping mechanism to break the circuit path.
The tripping systems described above are generally adequate for many simple circuit breaker applications, but there has been an increasing demand for a more intelligent and precise tripping system. For example, many businesses today use expensive 3-phase power equipment which provides critical functions to the business and its customers. Due to the cost of the equipment and the functions that the equipment provides, the power supplied to the equipment must be precisely measured and controlled. For this reason, processor-based tripping systems have been developed to attempt to provide programmable control to the equipment operator (user).
A major problem in the design of processor-based tripping systems has been to accurately and reliably measure the power provided to the equipment. On the other hand, small size and low cost are also desirable characteristics for the tripping systems. But the power measurement circuitry necessarily limits the size of the tripping system, and is also relatively expensive due to the component tolerances and circuit complexity required for precise current measurement.
Accordingly, in addition to requiring user-flexibility to power distribution systems, processor-based tripping systems must also accurately and reliably measure the current provided to the loads. Failing to perform in this manner often results in inadvertent (nuisance) trips or missed trips which may damage the equipment powered through the circuit breaker and the circuit breaker itself.